To be commercially sold in the United States a ground fault circuit interrupter or GFCI should preferably be able to pass testing performed in accordance with the Underwriter's Laboratory UL943 standard. UL943 requires, among other things, specific fault current levels and response timing requirements at which a GFCI should trip. For example, UL943 specifies that when a GFCI is powered up and operating normally, it should trip within approximately 25 ms when a fault of about 250 ma is applied to the GFCI. UL943 trip time requirements need to be satisfied not only within the time intervals between any simulating testing but also during the time period in which a simulated test is being carried out; i.e., where an external fault is encountered during any simulated testing procedure (including right before the start of the simulated test). An external fault is an actual fault or a fault not initiated by a self test.
The trip response time of a GFCI, and therefore by extension the duration of a simulated test procedure, may depend, at least in part, on the type of ground fault detection integrated circuit or IC employed. There are two types of commonly used ground fault detection ICs available commercially. The first type is available from, e.g., National Semiconductor (e.g., LM 1851) or Fairchild T Semiconductor (e.g., FAN1 851), in addition to other alternative vendors. The other type is available from, e.g., Fairchild Semiconductor (e.g., RV4141A), and is also available from other alternative vendors.
The first type of commonly used ground fault detection IC (i.e., the LM1 851, the FAN1 851, and the like) is an integrating type circuit and takes advantage of a timing curve specified in UL943 and as a result trips in accordance with the UL943 timing requirements for prescribed fault magnitudes. Use of the timing curve by ground fault detection ICs of this type generally results in a longer response time for smaller magnitude faults and a faster response time for larger magnitude faults. This variable response time is also exhibited by these types of ground fault detection ICs in the presence of simulated faults.
The other type of commonly used ground fault detection IC (i.e., the RV4141, and the like) which is a comparator circuit do not utilize the UL943 timing curve and instead have a generally small trip response time in the presence of a current level that exceeds a prescribed threshold. The typical trip response time for these types of ground fault detection ICs is 2 ms.
It is currently being considered to require automatic self testing of fault circuit interrupters.